Yeah, people keep trying to find a purely economic reason for why folks are having less kids. I think I'm satisfied with the very simple reason that raising kids is hard work. Most people feel like 1 kid already gets their hands full. That's really it IMO. Whenever it was that joint families split up into disparate nuclear families, obviously the load of raising say 5 kids that was being shared among 20 people, suddenly fell on just 2 people.
In the places with high fertility rate, you will almost surely observe joint families. I'm from India where we have such demographic variety that you can see adjacent areas with completely different fertility rates. In one, you will see old-style large houses with courtyards and 15+ people in them living as a joint family. Invariably these people have more kids. But in the next town with more nuclear families and modernish apartments, you will be lucky to see 2 kids per family. [1]
This is what births the secondary economic incentives which are mentioned a lot in popular discourse. For example, if you're already living in a house with 15 people your financial realities will require a similar number of people in the next generation to continue the same lifestyle.
[1] Wealth is not a confounding factor here. The specific two areas I have in mind are both more or less equally wealthy, one has folks running a coconut business primarily and the other is a small town with the usual assortment of office jobs.
Best simple and obvious explanation I’ve seen for the root cause.
So … probably most straightforward solution is to update local zoning laws to encourage a more communal style of living. Not communes but more along the lines of suburbs-as-village.
Or better child care provided by the government.
Having a child has a significant impact on your professional career (women in particular). Even in countries with great parental leave, like Denmark, the absence still reduces the years of experience you would otherwise get. Once they are "ready" for a child, they are in their late 30's and barely have time to birth one or two babies and would probably not have the stamina required to take on more.
The social pressure of raising the perfect kid with multiple activities in their "spare" time is much higher today and require more from the parent.
Ideally, people should get children in their early 20's, when they are physically more fit to handle the pressure. Unfortunately the current education system does not combine well with young parents. The government would need to integrate child care in colleges so that the parents have somewhere to place the children while studying.